1. Field of the Invention
The present invention relates to a process for the low pressure chemical vapor deposition (LPCVD) of metal silicide, especially molybdenum silicide, on a substrate at a low temperature.
2. Description of the Prior Art
In the past, integrated circuit devices have made extensive use of polysilicon layers, both as self-aligned gates for field-effect transistors and for electrical interconnects between various components. Traditionally, deposited polycrystalline silicon has been doped with conventional donor or acceptor elements to reduce its resistivity into the one milliohm-centimeter range. Since the film thickness is limited to about 0.5 micron maximum to permit patterning and reasonable step coverage, sheet resistances less than about 20 ohms per square are difficult to obtain. Such a value is too high for complex, high-performance integrated circuits because ohmic drops become too high.
In VLSI (very large scale integration) technology, highly doped polysilicon gate-contact material in MOS (metal oxide semiconductor) devices is replaced with a layer combination exhibiting a lower resistance, for example, one composed of a 300 nm thick doped polysilicon layer and a 200 nm thick metal disilicide layer wherein the metal is molybdenum, tungsten or tantalum. Metal silicides have attracted increasing attention recently as possible interconnect material for semiconductor integrated circuits. In particular, bilayer composite films of metal silicide and polycrystalline silicon have been found to provide low resistance gate and interconnect layers for MOS circuits and to be otherwise compatible with current silicon gate technology.
Such structures are commonly termed "polycide" structures. The sheet resistance of polycide structures is generally an order of magnitude lower than that of a single layer structure of doped polycrystalline silicon alone. Polycide structures are utilized to achieve a high conductivity, gate-level metallization for MOS devices. More particularly, polycide structures are utilized as gate material and interconnects in structure such as very high speed integrated circuits (VHSIC). Polycide structures have been used, for example, as the gate material in insulated gate field effect transistor (IGFET) devices and other similar structures.
Polycide structures are conventionally produced by depositing undoped polycrystalline silicon by LPCVD, doping the polycrystalline silicon, e.g., by diffusion using phosphorus oxychloride, and then depositing the refractory metal silicide, e.g., tantalum silicide, thereover by co-sputtering from separate targets of metal and silicon. Subsequent annealing is required to obtain the desired low conductivity of the silicide layer.
Tungsten disilicide has received particular attention as a candidate for such applications because its electrical resistivity is among the lowest of the metal silicides. In addition, tungsten disilicide grows a passivating silicon oxide layer if a silicon source is available and it is not attacked by HF solutions. Films of tungsten disilicide have been formed by sintering films of tungsten metal deposited on silicon by evaporation, sputtering or chemical vapor deposition (CVD). They have also been formed by sintering mixed films of tungsten and silicon formed by co-evaporation, co-sputtering, or sputtering from a tungsten disilicide target.
In an improvement on these methods, U.S. Pat. No. 4,359,490 describes a LPCVD process for co-depositing a metal, such as tungsten, molybdenum, tantalum and niobium, and silicon onto a surface such as a semiconductor integrated circuit. In this process, a LPCVD reactor is maintained at 500.degree.-700.degree. C. and at a pressure of 0.1-0.3 torr. Conventional CVD purging steps are utilized and silane is introduced into the reactor to deposit a base layer of polysilicon on the desired surface. Then tantalum chloride, for example, is introduced and a tantalum silicide layer is deposited on the surface at a rate of 100-300 angstroms/min.
Molybdenum disilicide films have also been deposited by LPCVD. See, for example, Inoue et al., J.Electrochem.Soc. 130, 1603 (1983). This process utilizes a hot wall procedure at 670.degree. C. The LPCVD of tungsten silicide has also been described by Brors et al., Solid State Technol., April 1983, p. 183; Saraswat el al., IEEE Trans.Elect.Dev. ED-30, 1497 (1983) and Brors et al. Proc. Electrochem.Soc. 1984, Chem. Vap. Depos., p. 275-286. The depositions described by the Brors et al. references were conducted at 350.degree.-450.degree. C. in a cold wall procedure. The LPCVD technique is currently preferred because it offers superior step coverage (over the sputtering technique), which is essential for state-of-the-art sub-micron devices.
U.S. Pat. No. 4,504,521 describes a similar process, but first deposits a doped amphorous silicon layer before depositing the tantalum silicide layer. According to this patent, a smooth surface of TaSi.sub.2 is deposited after annealing, which is not the case in U.S. Pat. No. 4,359,490.
An additional process for depositing a metal silicide has been described in U.S. Pat. No. 4,557,943. In this process, a film of titanium silicide which is substantially titanium disilicide is deposited by chemical vapor deposition by reacting a gaseous silicon species and a gaseous titanium species in a plasma. The preferred reactant species are silane (SiH.sub.4) and titanium tetrachloride (TiCL.sub.4) which are carried into a reaction chamber by an inert gaseous carrier such as argon. Since SiH.sub.4 and TiCl.sub.4 decompose at different temperatures, thermal decomposition is inadequate to produce films uniform composition and thickness over an extended reaction zone. In accordance with this invention, the decomposition reaction takes place in a plasma at relatively low temperature (450.degree. C.) by alternating electrical field, by a process generally termed plasma-enhanced chemical vapor deposition (PECVD). After deposition, the material is annealed at 600.degree.-700.degree. C.
In addition to the above art, additional reference can be made to Sinha, J.Vac. Sci.Technol. 19, 778 (1981); d'Heurle, Proc. Electrochem.Soc. 1982, VLSI Sci. Tech., pp 194-212; Crowder et al., IEEE Trans.Elect.Dev. ED-26, 369 (1979); Chow et al., IEEE Trans.Elect.Dev. ED-30, 1480 (1983); and Murarka, J.Vac.Sci. Technol. 17, 775 (1980), for discussions of refractory metal silicides.
One disadvantage of prior art LPCVD processes is a relatively poor adhesion of the metal silicide layer to the surface of the substrate. Another disadvantage is that the "low" temperatures are still too high for use of lift-off techniques and/or conformal metallization on plastics or other temperature-intolerant materials without rapid thermal processing (non-RTP); and the temperatures are also too high, resulting in unacceptable redistribution of dopants in sub-micron structures. Some attempts have been made to deposit molybdenum silicide by LPCVD at 300.degree.-400.degree. C. but have been unsuccessful, apparently as a result of excessive gas phase pre-reaction and impurities in the MoF.sub.6.